First, dinosaurs had feathers like birds. And now, scientists found that they weren't even cold-blooded like the pre-historic reptilian monsters we thought they were.
Decades ago, scientists used to think that dinosaurs were cold-blooded animals, like lizards and crocodiles. Cold-blooded or ectothermic animals rely on the environment to regulate their body temperature. That's why they become sluggish in cold weather, and don't have to eat as much to grow (snakes, for example, can eat just one meal a month to stay alive). In contrast, warm-blooded or endothermic animals, like mammals, generate internal heat to regulate the body's internal temperature. Mammals can grow fast and remain active in cold weather, but constantly maintaining their body's internal temperature requires a lot of energy.
That view began to change in 1968, when a young paleontologist named Robert T. Bakker suggested that <a href="http://www.ucmp.berkeley.edu/diapsids/metabolism.html"> dinosaurs were "fast, agile, energetic creatures"</a>, much like the warm-blooded mammals of today.
Fast forward to today, when a new research study published in the journal Science reported that dinosaurs were neither cold-blooded nor warm-blooded. They were like the Goldilocks of biological thermoregulation, as suggested by John Grady, an ecologist at the University of New Mexico, and colleagues. "You know, if you are a little bit ... warmer-blooded than a reptile," Grady said, "essentially your muscles fire faster; your nerves fire faster; you are a more dangerous predator."
Energy use of dinosaurs compared to ectothermic animals, such as crocodile and lizard, and endothermic ones like birds and modern mammals. (Image: John Grady)
For his research, Grady noted that warm-blooded animals grow faster than cold-blooded ones. Using technique developed by paleontologist Greg Erickson to determine growth rates of dinosaurs by analyzing their fossils, Grady and colleague were able to calculate the metabolic rates of these prehistoric animals.
"Our results showed that dinosaurs had growth and metabolic rates that were actually not characteristic of warm-blooded or even cold-blooded organisms. They did not act like mammals or birds nor did they act like reptiles or fish," said co-author Brian Enquist of University of Arizona. "Instead, they had growth rates and metabolisms intermediate to warm-blooded and cold-blooded organisms of today. In short, they had physiologies that are not common in today's world."
Grady characterized dinosaurs as "mesotherms" and attributed that unique physiology to the reason that dinosaurs could grow to be much bigger than mammals. "It is doubtful that a lion the size of T. rex could eat enough to survive," he said.